Dining the processing of a substrate in the plasma processing chamber, the substrate is typically clamped to a temperature-controlled chuck. Clamping is often performed using electrostatic clamping, which involves creating an electrostatic charge on the substrate to cause the substrate to be attracted to the electrostatic chuck. Clamping is an important component of substrate temperature control since proper clamping permits helium cooling of the back side of the substrate, to be properly controlled.
Following the processing step, the substrate needs to be dechucked. Dechucking involves removing the electrostatic charge from the substrate to remove the attraction force between the substrate and the electrostatic chuck. In most cases, the removal of the electrostatic charge from the substrate is accomplished by executing a dechucking plasma step whereby a plasma specifically formulated for the purpose of removing electrostatic charge from the substrate is employed. This plasma supplies the current to neutralize the electrostatic charge on the substrate. Following the electrostatic charge removal, lift pins disposed in the electrostatic chuck body may be employed to lift the substrate upward to separate the substrate from the electrostatic chuck surface, thereby allowing a robot arm to remove the substrate from the plasma processing chamber.
Proper dechucking is essential to the attainment of a high process yield and system throughput. If the electrostatic charges are not satisfactorily removed from the substrate, there is a possibility of wafer breakage when the lift pins attempt to lift up the substrate while the substrate is still clamped to the electrostatic chuck. When this happens, the substrate may be broken and/or otherwise destroyed, and a substantial amount of time and effort is then required to clean the substrate debris from the chamber. When the chamber is opened for cleaning, the entire plasma processing system is taken off the production line. Accordingly, substrate breakage due to improper dechucking is a costly occurrence for IC manufacturers and greatly increases the cost of tool ownership if allowed to occur.
Due to the severity of the consequences associated with improper dechucking, the dechucking plasma step in the prior art is often executed for a specified period of time duration, which may be determined using a best known method, or BKM, specification. The duration of the plasma dechucking step is usually fairly conservative (i.e., long in time duration) to ensure that the electrostatic charge is completely removed from the substrate. In the typical case, the dechucking plasma is run for the entire duration irrespective whether the electrostatic clamping charge has already been removed after only a few seconds. Following the expiration of the dechucking period, it is assumed that the substrate is ready to be lifted even if the electrostatic charge has not been satisfactorily removed. For this reason, the dechucking plasma duration tends to be very conservative in the prior art.
If satisfactory electrostatic charge removal occurs very early on in the plasma dechucking duration, the remaining portion of the plasma dechucking duration represents, in essence, wasted time since enough electrostatic charge has already been removed and no useful process occurs on the wafer and inside the plasma processing chamber during the remaining portion of the plasma dechucking process. The wasted time reduces the overall throughput of the plasma processing system, leading to a higher cost of ownership for the plasma tool (as a function of units of device produced). Furthermore, the presence of the dechucking plasma in the chamber during the wasted time duration contributes to the premature degradation of the chamber components (thereby necessitating more frequent cleaning and maintenance cycles) and/or contributes to the unwanted etching of the substrate without a corresponding benefit in terms of improved and/or increased substrate production.
On the other hand, if the electrostatic charge is not satisfactorily removed from the substrate after the expiration of the dechucking plasma duration, the initiation of the substrate removal step often results in broken substrates.
In view of the foregoing, there are desired improved techniques for detecting whether the dechucking is successful and for minimizing the time duration required to perform the dechucking step.